list manipulation - Reverse DeleteDuplicates using Information from Tally

I have a list of values, e.g., {1, 1, 1, 2, 2, 3, 3, 4, 4, 4, 4, 4, 4, 5}. I delete the duplicates with

DeleteDuplicates[{1, 1, 1 ,2, 2, 3, 3, 4, 4, 4, 4, 4, 4, 5}];

{1, 2, 3, 4, 5}

I want to perform some calculations on each value and at the end reverse the process of deleting.

From Tally I know, how often the elements appear:

Tally[{1, 1, 1, 2, 2, 3, 3, 4, 4, 4, 4, 4, 4, 5}];

{{1, 3}, {2, 2}, {3, 2}, {4, 5}, {5, 1}}

Now, from the calculations I have the new list {12, 14, 15, 16, 17}. I want to reverse the process of DeleteDuplicates on this list. Means: {{12,3},{14,2},{15,2},{16,5},{17,1}} -> So, that I get:

{12, 12, 12, 14, 14, 15, 15, 16, 16, 16, 16, 17}.

I want to do that, because the calculations take very long and I want to calculate duplicates two times.

Answer

tal = {{1, 3}, {2, 2}, {3, 2}, {4, 5}, {5, 1}};

lis = {12, 14, 15, 16, 17};

Flatten @ MapThread[Table[#1, {#2}] &, {lis, Last /@ tal}]

{12, 12, 12, 14, 14, 15, 15, 16, 16, 16, 16, 16, 17}

Flatten[ConstantArray @@@ Transpose[{lis, Last /@ tal}]]

Comments

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